Process for large-scale production of CdTe/CdS thin film solar cells

ABSTRACT

A process for large-scale production of CdTe/CdS thin film solar cells, films of the cells being deposited, in sequence, on a transparent substrate, the sequence comprising the steps of: depositing a film of a transparent conductive oxide (TCO) on the substrate; depositing a film of CdS on the TCO film; depositing a film of CdTe on the CdS film; treating the CdTe film with CdCl 2 ; depositing a back-contact film on the treated CdTe film. Treatment of the CdTe film with CdCl 2  comprises the steps of: forming a layer of CdCl 2  on the CdTe film by evaporation, while maintaining the substrate at room temperature; annealing the CdCl 2  layer in a vacuum chamber at a temperature generally within a range of 380° C. and 420° C. and a pressure generally within a range of 300 mbar and 1000 mbar in an inert gas atmosphere; removing the inert gas from the chamber so as to produce a vacuum condition, while the substrate is kept at a temperature generally within a range of 350° C. and 420° C. whereby any residual CdCl 2  is evaporated from the CdTe film surface.

FIELD OF THE INVENTION

The present invention relates generally to alternative energy resourcestechnology and, more particularly, to solar energy storage and the like.

BACKGROUND OF THE INVENTION

Conventional CdTe/CdS solar cells typically comprise a transparent glasssubstrate carrying a transparent conductive oxide (TCO) film, a CdS filmserving as the n-conductor, a CdTe film serving as the p-conductor and ametallic back-contact. A solar cell of this general description isdisclosed, for example, in U.S. Pat. No. 5,304,499, which issued on Apr.19, 1994.

While this “float” glass concept may be adapted for commercial use as atransparent substrate, frequent diffusion of Na into the TCO film hasoften resulted. Consequently, despite its relatively low cost, specialglasses are often preferred over such a “float” glass arrangement.

Perhaps the most common TCO material is In₂O₃ which contains about 10%Sn (ITO). This material is usually characterized by a very lowresistivity on the order of 3×10⁻⁴ Ωcm and a relatively hightransparency (>85%) in the visible spectrum. While useful this materialis made by sputtering and, after several runs, the ITO target formsnoodles which contain excess In. In addition, a discharge may occurbetween noodles during sputtering which can damage the film.

Another material that is commonly used for the transparent conductiveoxide film is fluorine doped SnO₂. Although helpful, this materialexhibits a higher resistivity close to about 10⁻³ Ωcm. As a result, a 1μm thick layer is necessary to keep the sheet resistance at about 10Ω/_(square). Generally, a high TCO thickness decreases the transparencyand, in turn, the photocurrent of the solar cell. In addition, a novelmaterial, namely Cd₂SnO₄, developed by the NREL group (X. Wu et al.,Thin Solid Films, 286 (1996) 274-276)) has been utilized. However, sincethe target is made up of a mixture of CdO and SnO₂, CdO being consideredhighly hygroscopic, the stability of the target has often been foundunsatisfactory.

Generally speaking, the CdS film is deposited either by sputtering orClose-Spaced Sublimation (CSS) from a CdS granulate material. The lattertechnique allows thin films to be prepared at a substrate temperatureconsiderably higher than that used in simple vacuum evaporation orsputtering. This is because the substrate and evaporation source arepositioned very close to one another, i.e., at a distance of 2-6 mm, anddeposition is performed in the presence of an inert gas such as Ar, Heor N₂ at a pressure of about 10⁻¹-100 mbar. A higher substratetemperature usually allows growth of a better quality crystallinematerial. A significant characteristic of close-spaced sublimation is avery high growth rate up to about 10 μm/min, which is suitable forlarge-scale production.

Next, a CdTe film is deposited on the CdS film through close-spacedsublimation at a substrate temperature of 480-520° C. CdTe granulate isgenerally used as a source of CdTe which is vaporized from an opencrucible.

An important step in the preparation of high efficiency CdTe/CdS solarcells is the treatment of CdTe film with CdCl₂. Traditionally, mostresearch groups would perform this step by depositing a layer of CdCl₂on top of CdTe by simple evaporation or by dipping the CdTe in amethanol solution containing CdCl₂, and then annealing the material inair at about 400° C. for between about 15 and about 20 min. It isgenerally believed that CdCl₂ treatment improves the crystalline qualityof CdTe by increasing the size of the small grains and removing defectsin the material.

After CdCl₂ treatment, the CdTe is etched in a solution of Br-methanolor in a mixture of nitric and phosphoric acid. Etching is necessary asCdO or CdTeO₃ are generally formed on the CdTe surface. CdO and/orCdTeO₃ must be removed in order to provide for good back-contact ontothe CdTe film. Also, it is believed that, since etching produces aTe-rich surface, formation of an ohmic contact when a metal is depositedon CdTe is facilitated.

The electric back-contact on the CdTe film is generally obtained bydeposition of a film of a highly p-dopant metal for CdTe such as copper,e.g., in graphite contacts, which, upon annealing, can diffuse in theCdTe film. Use of a Sb₂Te₃ film as a back-contact in a CdTe/CdS solarcell is set forth by applicants in N. Romeo et al., Solar EnergyMaterials & Solar Cells, 58 (1999), 209-218).

Industrial interest in thin films solar cells has increased in recentyears, especially in view of the relatively high conversion efficiencyachieved. Recently, for instance, a record 16.5% conversion efficiencywas reported (see X. Wu et al., 17^(th) European Photovoltaic SolarEnergy Conversion Conference, Munich, Germany, 22-26 Oct. 2001, II,995-1000). Accordingly, a number of attempts have been made to provideprocesses suitable for large-scale, in-line production of CdTe/CdS thinfilm solar cells. A state-of-the art report of these efforts may befound in D. Bonnet, Thin Solid Films 361-362 (2000), 547-552. Whilehelpful, they include crucial steps that also affect either thestability and efficiency of CdTe/CdS thin film solar cells or theircosts, thereby hindering achievement of a commercially viable process.

A significant problem of these processes is the etching step to whichthe CdTe surface must be submitted for removing CdO or CdTeO₃ oxidesthat form thereon. Although etching requires the steps of immersingsubstrates carrying the treated CdTe/CdS films into acid solutions,rinsing and drying, machinery suitable for such continuous operationdoes not currently exist. Another difficulty that can negatively affectthe stability of TCO films, as well as the cost of the final product,are the aforementioned disadvantages encountered using known TCOs.Moreover, known TCOs typically require the use of special glasses, suchas borosilicate glass, to avoid Na diffusion and associated damage tothe film that often occurs when soda-lime glass is used.

A further drawback of conventional processes relates to the source fromwhich the CdS film and the CdTe film are produced through close-spacedsublimation. When relatively small pieces of these materials whichcontain dust, are used as a sublimation source, because of a differentthermal contact, some micro-particles can overheat and, together withthe vapor, split onto the substrate. In an attempt to avoid thisinconvenience, complicated metallic masks are often used which makecontinuous operation problematic.

OBJECTS AND SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide aprocess suitable for large-scale production of stable and efficientCdTe/CdS thin film solar cells using a low cost substrate.

Another object of the present invention is to provide a process forlarge-scale production of stable and efficient CdTe/CdS thin film solarcells in which the CdTe film is treated with CdCl₂ so as to eliminatethe need for an etching step for removal of oxides that may form on theCdTe film.

It is a further object of the present invention to provide a process forlarge-scale production of stable and efficient CdTe/CdS thin film solarcells in which deposition of the TCO film is conducted such that a filmof very low resistivity can be deposited without formation of metalnoodles on the target, thereby enabling a relatively inexpensivesubstrate to be used.

Still another object of the present invention is to provide a processfor large-scale production of stable and efficient CdTe/CdS thin filmsolar cells which allows formation of CdS and CdTe films that areentirely free of dust.

It is yet another object of the present invention to provide a CdTe/CdSthin film solar cell that is stable, efficient and relatively low-cost.

According to one aspect of the present invention, treatment of CdTe filmwith CdCl₂ is accomplished by first forming a layer of CdCl₂ having athickness between about 100 and about 200 nm on the CdTe film byevaporation, while keeping the substrate at room temperature; thenannealing the CdCl₂ layer in a vacuum chamber at a temperature generallywithin a range of 380 and 420° C. and a pressure between about 300 andabout 1000 mbar under an inert gas atmosphere; and, finally, removingthe inert gas from the chamber so as to produce a vacuum condition,while the substrate is maintained at a temperature generally within arange of 350 and 420° C., whereby any residual CdCl₂ is evaporated fromthe CdTe film surface. As a result, etching treatment of the CdTe filmis unnecessary and the process can be conducted continuously.

According to another aspect of the present invention, the TCO layer isformed by sputtering in an inert gas atmosphere containing approximately1-3 vol. % hydrogen and a gaseous fluoroalkyle compound, in particular,CHF₃. In this manner, the TCO is doped with fluorine.

According to a further aspect of the present invention, as a sourcematerial for formation of the CdS and CdTe films by sputtering orclose-spaced sublimation, a CdS or CdTe material, respectively, is usedin the form of a relatively compact block.

Further features of the process according to the invention are set forthin the dependent claims.

In accordance with still another aspect of the present invention, aprocess is provided for large-scale production of CdTe/CdS thin filmsolar cells, films of the cells being deposited, in sequence, on atransparent substrate, the sequence comprising the steps of depositing afilm of a transparent conductive oxide (TCO) on the substrate;depositing a film of CdS on the TCO film; depositing a film of CdTe onthe CdS film; treating the CdTe film with CdCl₂; and depositing aback-contact film on the treated CdTe film. In treatment of the CdTefilm with CdCl, initially a layer of CdCl₂ is formed on the CdTe film byevaporation, while maintaining the substrate at room temperature.Second, the CdCl₂ layer is annealed in a vacuum chamber at atemperature, generally within a range of 380° C. and 420° C. and apressure generally within a range of 300 mbar and 1000 mbar in an inertgas atmosphere. Finally, the inert gas is removed from the chamber so asto produce a vacuum condition, while the substrate is kept at atemperature generally within a range of 350° C. and 420° C., whereby anyresidual CdCl₂ is evaporated from the CdTe film surface.

According to yet another aspect of the present invention, a process isprovided for large-scale production of CdTe/CdS thin film solar cells,films of the cells being deposited, in sequence, on a transparentsubstrate, the sequence comprising the steps of (i) depositing a film ofa transparent conductive oxide (TCO) on the substrate; (ii) depositing afilm of CdS on the TCO film; (iii) depositing a film of CdTe on the CdSfilm; (iv) treating the CdTe film with CDCl₂; and (v) depositing aback-contact film on the treated CdTe film, wherein the transparentconductive oxide is In₂O₃ doped with fluorine.

In accordance with still a further aspect of the present invention, aprocess for large-scale production of CdTe/CdS thin film solar cells isprovided. Films of the cells are deposited, in sequence, on atransparent substrate, the sequence comprising the steps of: depositinga film of a transparent conductive oxide (TCO) on the substrate;depositing a film of CdS on the TCO film; depositing a film of CdTe onthe CdS film; treating the CdTe film with CdCl₂; and depositing aback-contact film on the treated CdTe film, wherein as a source materialfor the formation of the CdS and the CdTe films by close-spacedsublimation, a CdS or CdTe material, respectively, generally in the formof a compact block is used.

Yet a further aspect of the present invention is directed to a CdTe/CdSthin film solar cell comprising a transparent substrate on which a layerof a transparent conductive oxide (TCO) is deposited. A CdS layer isdeposited on the TCO layer, a CdTe layer is deposited on the CdS layerand a back-contact layer on the CdTe layer, wherein the transparentconductive oxide is In₂O₃ doped with fluorine and the back-contact layeris formed by a layer of Sb₂Te₃ or As₂Te₃ covered by a layer of Mo.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects and advantages of the process for large-scaleproduction of CdTe/CdS thin film solar cells, according to the presentinvention, will become apparent from the following description ofspecific, illustrative embodiments thereof made with reference to thefollowing drawings, in which:

FIG. 1 is a schematic diagram showing a film deposition sequence forproducing CdTe/CdS thin film solar cells, according to one aspect of thepresent invention;

FIG. 2 is a flow diagram showing a process for producing CdTe/CdS thinfilm solar cells, according to another aspect of the present invention.

The same numerals are used throughout the drawing figures to designatesimilar elements. Still other objects and advantages of the presentinvention will become apparent from the following description of thepreferred embodiments.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings and, more particularly, to FIGS. 1-2,there is shown generally a specific, illustrative process for producingCdTe/CdS solar cells, according to the present invention. Such solarcells comprise five layers deposited in a selected sequence on atransparent base layer or substrate, the layers consisting of about a300 nm to about a 500 nm thick layer of a transparent conducting oxide(TCO), about an 80 nm to about a 200 nm thick layer of CdS deposited onthe TCO layer, about a 4 μm to about a 12 μm thick layer of CdTe on theCdS layer and a back contact layer formed of at least about a 100 nmthick layer of SB₂Te₃ and 100 nm thick layer of Mo. In particular, thetransparent base substrate preferably comprises soda-lime glass and thetransparent conducting oxide is desirably fluorine-doped (In₂O₃.F).

TCO layer consists of In₂O₃, which is doped with fluorine during growth.The In₂O₃ target, unlike ITO, does not form any noodles. A very lowresistivity is obtained by introducing a relatively small amount offluorine to the sputtering chamber in the form of a gaseous fluoroalkylecompound such as CHF₃ and a small amount of H₂ in the form of a mixturewith an inert gas such as a Ar+H₂ mixture, in which H₂ is around 20% inrespect to Ar. A typical example is a generally 500 nm thick film ofIn₂O₃ deposited with a deposition rate generally higher than about 10Å/sec at a substrate temperature of approximately 500° C., with an Arflow-rate of about 200 sccm, a CHF₃ flow-rate of roughly 5 sccm and anAr+H₂ flow-rate of around 20 sccm. Accordingly, the reactive sputteringgas comprises Ar in the amount of about 2.5 vol. % of CHF₃ and about 1.8vol. % of H₂. This film exhibits a sheet resistance of approximately 5Ω/_(square), a resistivity of 2.5×10⁻⁴ Ωcm and a transparency higherthan about 85% in the wavelength generally within a range of 400 and 800nm. Another characteristic of this film is its high degree of stabilityas well as its ability to prevent Na diffusion from the soda-lime glass.This has been demonstrated by making CdTe/CdS solar cells on top of thistype of TCO which have shown to be very stable, even if heated to about180° C. when illuminated by “ten suns” for several hours.

After deposition of the CdS and CdTe films in the known way ofsputtering or close-spaced sublimation, the CdTe film surface is treatedwith CdCl₂ as follows.

First, 200 nm of CdCl₂ are deposited by evaporation on top of CdTe filmwith the substrate kept at room temperature. Annealing is then performedfor about 15-20 min at approximately 400° C. in a vacuum chamber inwhich roughly 500 mbar of Ar is introduced. After annealing, the chamberevacuated while keeping the substrate at a temperature of about 400° C.for 5 min. Since CdCl₂ has a relatively high vapor pressure at about400° C., any residual CdCl₂ re-evaporates from the CdTe surface. It isnoted that CdO or CdTeO, are not formed since annealing is conducted inan inert atmosphere, which does not generally contain O₂.

According to various aspects of the present invention, a Te-rich surfaceis not needed to obtain a non-rectifying contact if the contact is madeby depositing a thin layer of a relatively highly conducting p-typesemiconductor, such as Sb₂Te₃ or As₂Te₃, on top of the CdTe film.Generally speaking, a good, non-rectifying contact is achieved on aclean CdTe surface if a layer of Sb₂Te₃ or As₂Te₃ at least about 100 mmthick is deposited by sputtering at a substrate temperature of betweenabout 250 and about 300° C. and between about 200 and about 250° C.,respectively. Sb₂Te₃ grows naturally as p-type with a resistivity ofaround 10⁻⁴ Ωcm, whereas As₂Te₃ also grows as p-type but with aresistivity of roughly 10⁻³ Ωcm. The contact procedure is then completedby covering the low resistivity p-type semiconductor with at least about100 nm of Mo₂ as is considered common practice in the art. A relativelythin layer of Mo is necessary in order to have a relatively lowsheet-resistance on the back-contact.

As a source of CdS and CdTe materials used to form the respective layersby sputtering or CSS, a granulate material can be used, as is commonpractice in the art. However, in view of the disadvantages indicatedabove, when operating in this way, according to a preferred embodimentof the present invention, a new sublimation source can be used whichconsists of a relatively compact block obtained by melting andsolidifying the material in an oven capable of sustaining a temperaturegenerally higher than the melting temperature of the material.

A procedure for preparing the CdS compact block is as follows: pieces ofCdS are put in a graphite container of the desired volume together withboron oxide (B₂O₃), which is a low melting point material (450° C.) andexhibits a very low vapor pressure when melted. Since boron oxide has adensity lower than that of CdS in the molten state, it floats over theCdS and covers the CdS completely upon cooling. In this way, CdS coveredwith B₂O₃, if it is placed in an oven containing an inert gas at apressure higher than approximately 50 atm. does not evaporate even at atemperature higher than its melting point. Since CdS melts at atemperature of about 1750° C., the oven is heated up to a temperature ofabout 1800° C. or more, and then cooled down to room temperature. Inthis manner, a unique compact block of CdS is obtained that isparticularly suitable for use as a sublimation source in a close-spacedsublimation system. CdS films prepared using this type of sourceresulted in a very smooth film that is completely free of dust. The CdSfilms used to prepare the CdTe/CdS solar cells are typicallyapproximately 100 nm thick. The substrate temperature is preferably keptbetween about 200 and 300° C. when CdS is prepared by sputtering andgenerally within a range of 480 and 520° C. when it is prepared byclose-spaced-sublimation. The sputtered CdS layer generally requiresannealing at about 500° C. in an atmosphere containing O₂ in order forthe CdS/CdTe solar cell to exhibit a relatively high efficiency. In thecase where CdS is prepared by close-spaced sublimation, O₂ is introducedto the sublimation chamber during deposition. While the role of O₂ isnot known, it is presumed to passivate the CdS grain boundaries.

In accordance with another embodiment of the present invention, the CdTesource too is a generally compact block obtained through melting andsolidifying pieces of CdTe in an oven under high pressure, as describedpreviously. Since CdTe melts at around 1120° C., the oven must be heatedto about 1200° C. in order to have complete melting of the CdTe pieces.CdTe films are deposited on the CdS by close-spaced sublimation at asubstrate temperature generally within a range of 480 and 520° C.Deposition rates during CdTe growth are typically about 4 μm/min. Inthis manner, about 8 μm of CdTe are deposited in approximately 2minutes.

By following the procedure described above, several solar cells havebeen prepared using as a substrate a I inch square low-cost soda-limeglass. A typical area of these cells is 1 cm². The finished cells aregenerally put under 10-20 suns for several hours at a temperature ofaround 180° C. in the open-circuit-voltage (V_(oc)) conditions.Advantageously, an increase in efficiency of about 20% or more is,thereby, achieved and without any material degradation.

The efficiency of these cells is generally within a range of 12% and 14%with open-circuit-voltages (V_(oc)) larger than approximately 800 mV,short-circuit-currents (J_(ac)) of between about 22 and about 25 MA/cm²and fill-factors (ff) ranging from about 0.6 to about 0.66.

EXAMPLE

A cell exhibiting an efficiency of around 14% has been prepared in thefollowing way: soda-lime glass is covered with 500 nm of In₂O₃.F(fluorine-doped) deposited at a substrate temperature of about 500° C.as described above. 100 nm of CdS are then deposited thereon bysputtering at approximately 300° C. substrate temperature and annealedfor about 15 min. at approximately 500° C. in 500 mbar of Ar containingabout 20% O₂. 8 μm of CdTe are deposited on top of the CdS by CSS at asubstrate temperature of approximately 500° C. Both the CdS and CdTefilms are produced from a relatively compact block source as describedabove. A treatment with 150 nm of CdCl₂ is then performed in an Aratmosphere, as described above. Finally, a back-contact is created,without any etching, by depositing, in sequence, through sputteringabout 150 nm of Sb₂Te₃ and 150 nm of Mo.

After one hour under 10 suns at a temperature of about 180° C. inopen-circuit conditions, the solar cell prepared in this way exhibitedthe following characteristics:

V_(OC) 852 mv J_(SC) 25 mA/cm² ff 0.66 efficiency 14%

The techniques used in this process, such as sputtering and close-spacedsublimitation, are fast, reproducible and easily scalable.

In general, sputtering systems capable of covering an area of around1×0.5 m² of glass are already commercially available, while close-spacedsublimation, which at a laboratory scale can readily cover 20×20 cm²area glass, does not yield any problems in being scaled up to about1.5×0.5 m². An in-line process can work well if a large area glass ismoved slowly over the different sources, namely, TCO, CdS, CdTe, Sb₂Te₃or As₂Te₃, and Mo. In order to connect the single cells in series, thein-line system desirably also includes three laser scribing processes,the first one after the TCO deposition, the second one before theback-contact deposition, and the third at the end of the process. Anrelatively important part of the process, according to the presentinvention, is that no acids or liquids are used and, as a consequence,the process can be conducted on a continuous basis without interruptionas is often needed to for etching in acid or in a Br-methanol solution.

Various modifications and alterations to the invention may beappreciated based on a review of this disclosure. These changes andadditions are intended to be within the scope and spirit of theinvention as defined by the following claim.

1. A process for large-scale production of CdTe/CdS thin film solarcells, films of the cells being deposited, in sequence, on a transparentsubstrate, the sequence comprising the steps of: depositing a film of atransparent conductive oxide (TCO) on the substrate; depositing a filmof CdS on the TCO film; depositing a film of CdTe on the CdS film;treating the CdTe film with CdCl₂; and depositing a back-contact film onthe treated CdTe film; wherein the treatment of the CdTe film with CdCl₂comprises the steps of: forming a layer of CdCl₂ on the CdTe film byevaporation, while maintaining the substrate at room temperature;annealing the CdCl₂ layer in a vacuum chamber at a temperature generallywithin a range of 380° C. and 420° C. and a pressure generally within arange of 300 mbar and 1000 mbar in an inert gas atmosphere; and removingthe inert gas from the chamber so as to produce a vacuum condition,while the substrate is kept at a temperature generally within a range of350° C. and 420° C. whereby any residual CdCl₂ is evaporated from theCdTe film surface.
 2. The process set forth in claim 1, wherein theCdCl₂ layer is between about 100 nm and about 200 nm thick.
 3. Theprocess set forth in claim 1, wherein annealing of the CdCl₂ layer iscarried out for about 15-20 minutes.
 4. The process set forth in claim1, wherein the inert gas is Ar.
 5. The process set forth in claim 1,wherein the back-contact film is formed of a Sb₂Te₃ layer covered by alayer of Mo.
 6. The process set forth in claim 5, wherein the Sb₂Te₃layer is formed by sputtering at a temperature between about 250° C. andabout 300° C.
 7. The process set forth in claim 1, wherein theback-contact film is formed of a As₂Te₃ layer covered with a layer ofMo.
 8. The process set forth in claim 7, wherein the As₂Te₃ layer isformed by sputtering at a temperature between about 200° C. and about250° C.
 9. The process set forth in claim 1, wherein the transparentconductive oxide is In₂O₃ doped with fluorine.
 10. The process set forthin claim 9, wherein the TCO layer is formed by sputtering in an inertgas atmosphere comprising a mixture of hydrogen and a gaseousfluoroalkyle compound.
 11. The process set forth in claim 10, wherein amixture of Ar and hydrogen is used which comprises between about 1% andabout 3% hydrogen by volume, and wherein the fluoroalkyle compound isCHF₃.
 12. The process set forth in claim 1, wherein, as a sourcematerial for formation of the CdS and the CdTe films by close-spacedsublimation, a CdS or, respectively, CdTe material generally in the formof a compact block is used.
 13. The process set forth in claim 12,wherein the compact block CdS (or CdTe) material is formed by coveringpieces of CdS (or CdTe) material with boron oxide, heating the coveredmaterial to a temperature generally greater than its melting point in aninert gas atmosphere and at a pressure generally greater than 50 atm,and then cooling the material to room temperature, whereby the materialis solidified in a generally compact block-like form.
 14. A process forlarge-scale production of CdTe/CdS thin film solar cells, films of thecells being deposited in sequence, on a transparent substrate, thesequence comprising the steps of: depositing a film of a transparentconductive oxide (TCO) on the substrate; depositing a film of CdS on theTCO film; depositing a film of CdTe on the CdS film; treating the CdTefilm with CdCl₂; and depositing a back-contact film on the treated CdTefilm; wherein the transparent conductive oxide is In₂O₃ doped withfluorine.
 15. The process set forth in claim 14, wherein the TCO layeris formed by sputtering in an inert gas atmosphere comprising a mixtureof hydrogen and a gaseous fluoroalkyle compound.
 16. The process setforth in claim 15, wherein a mixture of Ar and hydrogen is used whichcomprises between about 1% and about 3% hydrogen by volume, and whereinthe fluoroalkyle compound is CHF₃.
 17. A process for large-scaleproduction of CdTe/CdS thin film solar cells, films of the cells beingdeposited, in sequence, on a transparent substrate, the sequencecomprising the steps of: depositing a film of a transparent conductiveoxide (TCO) on the substrate; depositing a film of CdS on the TCO film;depositing a film of CdTe on the CdS film; treating the CdTe film withCdCl₂; and depositing a back-contact film on the treated CdTe film;wherein as a source material for the formation of the CdS and the CdTefilms by close-spaced sublimation, a CdS or, respectively, CdTe materialgenerally in the form of a compact block is used.
 18. The process setforth in claim 17, wherein the compact block of CdS (or CdTe) materialis formed by covering pieces of CdS (or CdTe) material with boron oxide,heating the covered material to a temperature generally greater than itsmelting point in an inert gas atmosphere and at a pressure generallygreater than about 50 atm, and then cooling the material to roomtemperature, whereby the material is solidified in a generally compactblock-like form.
 19. The process set forth in claim 1, wherein thetransparent substrate is soda-lime glass.